Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/resolution/members.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.resolution.members;
 
 import '../common.dart';
 import '../common/names.dart' show Selectors;
 import '../common/resolution.dart' show Resolution;
 import '../compile_time_constants.dart';
@@ skipping 1625 matching lines @@
 
     // TODO(23998): Remove these when all information goes through
     // the [SendStructure].
     registry.useElement(node, target);
     registry.setSelector(node, selector);
     return const NoneResult();
   }
 
   /// Handle a [Send] whose selector is an [Operator], like `a && b`, `a is T`,
   /// `a + b`, and `~a`.
+  // ignore: MISSING_RETURN
   ResolutionResult handleOperatorSend(Send node) {
     String operatorText = node.selector.asOperator().source;
     if (operatorText == 'is') {
       return handleIs(node);
     } else if (operatorText == 'as') {
       return handleAs(node);
     } else if (node.arguments.isEmpty) {
       UnaryOperator operator = UnaryOperator.parse(operatorText);
       if (operator == null) {
         return handleUnresolvedUnary(node, operatorText);
@@ skipping 3140 matching lines @@
     }
     return const NoneResult();
   }
 }
 
 /// Looks up [name] in [scope] and unwraps the result.
 Element lookupInScope(
     DiagnosticReporter reporter, Node node, Scope scope, String name) {
   return Elements.unwrap(scope.lookup(name), reporter, node);
 }